Chronic fatigue syndrome (CFS) is a serious health problem in the United States, affecting at least 3 of every 1000 patients seen in general medicine clinics. No effective treatment has been identified for the disorder. Recent observations suggest a strong association between CFS and a treatable disorder in the regulation of blood pressure known as neurally mediated hypotension. In small, unblinded studies, treatment with fludrocortisone and other medications directed against neurally mediated hypotension has appeared to be beneficial for patients with CFS, with 40% of patients reporting an almost complete resolution of symptoms, and another 30% reporting some improvement. The specific aim of this randomized, double blind, placebo controlled trial is to determine in an unbiased fashion whether fludrocortisone is efficacious for those with CFS. The study will randomize 100 adults with CFS to receive either fludrocortisone or placebo. The primary hypothesis to be tested is that within two months of starting the study medication, 30% more individuals treated with fludrocortisone than with placebo will experience a clinically important 20 point improvement (out of 100) in their general sense of well being. A secondary hypothesis is that symptomatic improvement will correlate with an improvement of one stage in the time until hypotension is provoked during a tilt table test. Blinded treatment will continue for nine weeks. After outcome assessment at nine weeks, patients will be observed off medication for two weeks. Patients are eligible for the study if they are aged 18-49, satisfy the 1994 CDC criteria for CFS, have undergone a medical evaluation to exclude other causes of CFS, and have hypotension provoked during stage 1 or stage 2 of an upright tilt test, performed as part of the study.